There is only so much fuel/ air mixture an engine can draw through the carb and into the cylinder. If the reducer is such a size that this maximum has not been reached then a larger reducer will allow more air into the carb and hence get closer to the max the engine can handle given it's capacity and port timing's. More power!
So if a 6mm reducer restricts this flow. Going to a 7mm will give you more fuel/air and more power. If 7mm give's the engine the maximum air and hence fuel/air mixture available to be drawn into the engine. Then going to an 8mm reducer will not do much. No extra power.
Again. This is given the same port timings and sizes in the engine as these could well then be the restriction the engine has against attaining it's max power output.

Don't forget. Extra air passing through your carb will need more fuel ! So the bigger reducer will cause a greater fuel consumption.

Hope this helps.

Regards
BM

Maybe I am wrong but by racing rules the hole in a .12 carb is limited to 5.5mm. Drilling it up to 6mm can be dune but up to 8mm...?

As previously stated by others you are more than likely not going to gain anything by running a carb reducer past the recommended manufacturer specs. Your engine is just going to burn more fuel, more inefficiently. What is the actual carb I.D. ? There is only so much you are able to do with a motor, and some times diving off the deepend just creates more problems...

Are we talking about a .12 or a .21 engine???

Again, no normal .12 engine has an carburator larger than 5.5mm, even if you put in an 9mm restrictor the hole will still have a maximum opening of 5.5mm.